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</head>
<body class="article toc2 toc-left">
<div id="header">
<h1>Snowflakes control sample</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_before_we_start">Before we start</a>
<ul class="sectlevel2">
<li><a href="#_when_to_render_on_your_own">When to Render on your own</a></li>
<li><a href="#_defining_the_game_concept">Defining the game concept</a></li>
</ul>
</li>
<li><a href="#_the_solution">The Solution</a>
<ul class="sectlevel2">
<li><a href="#_step_1_prepare_the_snowflake_model">Step 1: Prepare the Snowflake-Model</a></li>
<li><a href="#_step_2_set_up_the_gameviewmodel">Step 2: Set up the GameViewModel</a></li>
<li><a href="#_step_3_add_the_snowflakescontrol">Step 3: Add the SnowflakesControl</a></li>
<li><a href="#_step_4_add_the_control_to_the_view">Step 4: Add the control to the view</a></li>
<li><a href="#_bonus_1_implement_a_custom_draw_operation">Bonus 1: Implement a custom draw operation</a></li>
<li><a href="#_bonus_2_adding_some_beauty_to_the_game">Bonus 2: Adding some beauty to the game</a></li>
<li><a href="#_see_it_in_action">See it in action</a></li>
</ul>
</li>
<li><a href="#_related">Related</a></li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>This example will show you how to create a custom control that overrides <code>OnRender</code> in order to have advanced render capabilities. It also shows how custom hit-testing can help to solve special tasks. We will create a simple game where the user should collect some snowflakes with their mouse pointer.</p>
</div>
<!-- toc disabled -->
<h3 id="_difficulty" class="discrete">Difficulty</h3>
<div class="paragraph">
<p>🐔 Normal 🐔</p>
</div>
<h3 id="_buzz_words" class="discrete">Buzz-Words</h3>
<div class="paragraph">
<p>Custom Control, Render, Hit-Testing, Custom Animations</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_before_we_start">Before we start</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In this sample we assume that you are familiar with Avalonia, the <code>MVVM</code>-pattern and how to create custom <code>Controls</code>, as well as how to define <code>AvaloniaProperties</code>. If this is new to you, please start with other tutorials first:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="../../../../README.html#_mvvm_samples">[MVVM-Basics]</a></p>
</li>
<li>
<p><a href="../../CustomControls/RatingControlSample/README.html">[RatingControlSample]</a></p>
</li>
<li>
<p><a href="https://docs.avaloniaui.net/docs/guides/custom-controls/">[Avalonia-Documentation]</a></p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_when_to_render_on_your_own">When to Render on your own</h3>
<div class="paragraph">
<p>There may be some special cases where a <code>TemplatedControl</code>, <code>ContentControl</code> or similar base class is not well suited, for example:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>If you need to display complex geometry</p>
</li>
<li>
<p>If you have many objects to render which otherwise adds a huge layout-overhead</p>
</li>
<li>
<p>If you need to control each render step</p>
</li>
<li>
<p>if you want to use special things otherwise not available, such as <a href="https://learn.microsoft.com/en-us/dotnet/api/skiasharp?view=skiasharp-2.88">[custom SkiaSharp-drawings]</a></p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_defining_the_game_concept">Defining the game concept</h3>
<div class="paragraph">
<p>Before we finally start to implement the game, we should make some considerations how our control should look like and how it should work. To shorten it, we use a plain list of notes:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The game will have just one level</p>
</li>
<li>
<p>The game must not run infinite &#8594; We need a way to start and stop the game</p>
</li>
<li>
<p>The game must render several snowflakes (represented as circles)</p>
<div class="ulist">
<ul>
<li>
<p>Each snowflake should have a random start position</p>
</li>
<li>
<p>Each snowflake should be able to move with different speed</p>
</li>
<li>
<p>Snowflakes should be of different sizes</p>
</li>
</ul>
</div>
</li>
<li>
<p>The user should be able to collect some score</p>
<div class="ulist">
<ul>
<li>
<p>The user must be able to collect snowflakes with their mouse device</p>
</li>
<li>
<p>Nice to have: The user should be able to see their current score</p>
</li>
<li>
<p>Nice to have: Show a score indicator for each hit snowflake</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_the_solution">The Solution</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We will create a custom control that will draw itself based on the data provided.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
To show you how this control can be bound to a <code>ViewModel</code> we will use the <code>MVVM</code>-pattern for demonstration here, even if it is not really needed in our case.
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="_step_1_prepare_the_snowflake_model">Step 1: Prepare the Snowflake-Model</h3>
<div class="paragraph">
<p>In the folder <code>Models</code> we create a new file <code>Snowflake.cs</code> which will have the following properties:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 1. Public properties of the snowflake class</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Property</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">X</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the horizontal position in relative unit (0 &#8230;&#8203; 1)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Y</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the vertical position in relative unit (0 &#8230;&#8203; 1)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Diameter</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the diameter in pixel</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Radius</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the radius in pixel</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Speed</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the peed of the snowflake in 1/second (relative unit)</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>As we want the control to be of any size and resizable, we store the position in relative units which we need to convert into the actual viewport on request.</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
In Avalonia you can use <code>Bounds</code> to get the Controls bounds, which we here call "viewport"
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>To achieve this conversion we add method that takes a <code>Rect</code> as parameter and returns a <code>Point</code> positioned inside this rectangle.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public Point GetCenterForViewport(Rect viewport)
{
    return new Point(X * viewport.Width + viewport.Left,
        Y * viewport.Height + viewport.Top);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now that would be enough to render the snowflakes, but these should also be able to move. Since speed is in <code>1/second</code> we can use the elapsed time between two frames to calculate the new position.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public void Move(double elapsedMilliseconds)
{
    // Speed is in 1/sec so we have to convert it into 1/ms.
    Y += elapsedMilliseconds * Speed / 1000.0;

    if (Y &gt; 1)
    {
        Y = 0;
    }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Since we also want the snowflake to be hit-able, we need a way to test whether a certain point is inside or outside our bounds. As we represent the snowflake as circles, this is straight forward:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public bool IsHit(Point point, Rect viewport)
{
    // since the snowflake is represented as a circle, we just can test
    // if the distance to the center is equal of smaller than the radius.

    var distance = ((Vector)(GetCenterForViewport(viewport) - point)).Length;
    return distance &lt;= Radius;
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Last but not least the hit-score can be calculated in the following way:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public int GetHitScore()
{
    // The smaller and the faster the snowflake, the higher the score.
    return (int)(1/Radius * 200 + Speed / 10.0);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Here is the final class <a href="Models/Snowflake.cs">Models/Snowflake.cs</a></p>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_set_up_the_gameviewmodel">Step 2: Set up the GameViewModel</h3>
<div class="paragraph">
<p>The <code>ViewModel</code> will be responsible for creating and managing the snowflakes, tracking the game status and storing the users current score.</p>
</div>
<div class="paragraph">
<p>It has the following public properties:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 2. Public properties of the SnowflakeGameViewModel-class</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Property</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Snowflakes</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">ObservableCollection&lt;Snowflake&gt;</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets a collection of available snowflakes.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">GameDuration</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">TimeSpan</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets the duration of the game. Also updates GameDurationMilliseconds.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">GameDurationMilliseconds</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets duration of the game in milliseconds.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">MillisecondsRemaining</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets how many times is remaining in milliseconds.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">IsGameRunning</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">bool</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets if the game is currently running.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Score</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets the current users score.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>In addition, we need these two private fields:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 3. Private fields of the SnowflakeGameViewModel-class</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Field</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">_gameTimer</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">DispatcherTimer</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">A readonly timer that ticks every 100 ms and updates the game status. Only active if the game is running.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">_stopwatch</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Stopwatch</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">A readonly stopwatch to measure elapsed game time. Will be reset when the game ends.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>To start a new game we add a <a href="../../MVVM/CommandSample/README.html">[Command]</a> which will clear any previous snowflakes, add some new snowflakes with random parameters and start the game:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">[RelayCommand]
private void StartGame()
{
    // Clear all snowflakes.
    Snowflakes.Clear();

    // Reset game score.
    Score = 0;

    // Add some snowflakes at random positions, having random diameters and speed.
    for (int i = 0; i &lt; 200; i++)
    {
        Snowflakes.Add(new Snowflake(
            Random.Shared.NextDouble(),
            Random.Shared.NextDouble(),
            Random.Shared.NextDouble() * 5 + 2,
            Random.Shared.NextDouble() / 5 + 0.1));
    }

    // Set game time.
    GameDuration = TimeSpan.FromMinutes(1);

    // Indicate that game has started.
    IsGameRunning = true;

    // Start the timer and stopwatch.
    _stopwatch.Restart();
    _gameTimer.Start();
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Finally, we need to update the game status whenever the game timer ticks. This is done in this handler:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">private void OnGameTimerTick(object? sender, EventArgs e)
{
    // Update the remaining milliseconds left.
    OnPropertyChanged(nameof(MillisecondsRemaining));

    // If no more time is left, stop the timer and stopwatch and stop the game.
    if (MillisecondsRemaining &lt;= 0)
    {
        _gameTimer.Stop();
        _stopwatch.Stop();
        IsGameRunning = false;
    }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Here is the final class: <a href="ViewModels/SnowflakeGameViewModel.cs">ViewModels/SnowflakeGameViewModel.cs</a></p>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_add_the_snowflakescontrol">Step 3: Add the SnowflakesControl</h3>
<div class="paragraph">
<p>Now it&#8217;s time to add the needed <code>Control</code> to render our game. Therefore, we add a new folder <code>Controls</code> and inside we add a new class <code>SnowflakesControl.cs</code>. This class must inherit <code>Control</code>. In addition, we want to implement the interface <code>ICustomHitTest</code> in order to control hit-testing on our own.</p>
</div>
<div class="paragraph">
<p>The control needs some <a href="https://docs.avaloniaui.net/docs/guides/custom-controls/how-to-create-advanced-custom-controls">[AvaloniaProperties]</a> to allow us to bind to it.</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 4. Public bindable properties of the SnowflakesControl-class</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Property</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Snowflakes</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">IList&lt;Snowflake&gt;</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets a list of snowflakes to render. This is a direct property.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">IsRunning</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">bool</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets if the game is running. This is a styled property.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Score</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">double</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets or sets the users score. This is a direct property.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>All the above properties should trigger a new render pass if they change. We can du this by adding the following line into the static constructor:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">static SnowflakesControl()
{
    // Make sure Render is updated whenever one of these properties changes
    AffectsRender&lt;SnowflakesControl&gt;(IsRunningProperty, SnowflakesProperty, ScoreProperty);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>In addition, we have one private field as follows:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 5. Private fields of the SnowflakesControl-class</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Field</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">_stopwatch</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Stopwatch</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">A readonly stopwatch to measure elapsed between two render frames. Will be reset after each render frame.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>The stopwatch will be started or stopped whenever <code>IsRunning</code> changed. We can do this by overriding <code>OnPropertyChanged</code> and filter for the property of interest accordingly:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
{
    base.OnPropertyChanged(change);

    // If IsRunning is updated, we need to start or stop the stopwatch accordingly.
    if (change.Property == IsRunningProperty)
    {
        if (change.GetNewValue&lt;bool&gt;())
        {
            // Resets and starts the stopwatch
            _stopwatch.Restart();
        }
        else
        {
            _stopwatch.Stop();
        }
    }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>This is all we need to start rendering. If you override <code>OnRender</code>, you will get a <code>DrawingContext</code> that has several methods available to render low-level geometry. So let&#8217;s do that and render our snowflakes:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-cs" data-lang="cs">public override void Render(DrawingContext context)
{
    // Figure out how much time elapsed since last render loop
    var elapsedMilliseconds = _stopwatch.Elapsed.TotalMilliseconds;

    foreach (var snowFlake in Snowflakes)
    {
        // If the game is running, move each flake to it's updated position
        if (IsRunning)
        {
            snowFlake.Move(elapsedMilliseconds);
        }

        // Draw the snowflake (we use a simple circle here)
        context.DrawEllipse(
            Brushes.White,
            null,
            snowFlake.GetCenterForViewport(Bounds),
            snowFlake.Radius,
            snowFlake.Radius);
    }

    base.Render(context);

    // Request next frame as soon as possible, if the game is running. Remember to reset the stopwatch.
    if (IsRunning)
    {
        Dispatcher.UIThread.Post(InvalidateVisual, DispatcherPriority.Background);
        _stopwatch.Restart();
    }
}</code></pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
We use <code>Dispatcher.UIThread.Post</code> to request the next frame in order to give the UI-thread enough time to handle input etc. If we didn&#8217;t use the dispatcher, the app would freeze.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>This is how a single render frame would look like:</p>
</div>
<div class="paragraph">
<div class="title">Result of a single render loop</div>
<p><span class="image"><img src="_docs/single_render_frame.png" alt="single render frame"></span></p>
</div>
<div class="paragraph">
<p>As we wanted to implement <code>ICustomHitTest</code>, we will add the needed interface members, which is just the following method here:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public bool HitTest(Point point)
{
    if (!IsRunning) return false;

    var snowFlake = Snowflakes.FirstOrDefault(x =&gt; x.IsHit(point, Bounds));
    if (snowFlake != null)
    {
        Snowflakes.Remove(snowFlake);
        Score += snowFlake.GetHitScore();
    }

    return snowFlake != null;
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>What this method does is:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>If the game is not running, the control shouldn&#8217;t receive any hit</p>
</li>
<li>
<p>If the game is running, search for any snowflake hit by the pointer</p>
<div class="ulist">
<ul>
<li>
<p>If one is found, remove it from the collection</p>
</li>
<li>
<p>If one is found, add the score to the total score</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p>Here is the final class <a href="Controls/SnowflakesControl.cs">Controls/SnowflakesControl.cs</a></p>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_add_the_control_to_the_view">Step 4: Add the control to the view</h3>
<div class="paragraph">
<p>To use the control, just add it to any sample view.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
Remember to add the needed xml-namespaces
</td>
</tr>
</table>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:controls="clr-namespace:SnowflakesControlSample.Controls"
             xmlns:viewModels="clr-namespace:SnowflakesControlSample.ViewModels"
             mc:Ignorable="d"
             d:DesignWidth="800"
             d:DesignHeight="450"
             x:Class="SnowflakesControlSample.Views.MainView"
             x:DataType="viewModels:SnowflakeGameViewModel"&gt;

    &lt;Design.DataContext&gt;
        &lt;!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) --&gt;
        &lt;viewModels:SnowflakeGameViewModel /&gt;
    &lt;/Design.DataContext&gt;

    &lt;Panel&gt;
        &lt;!-- Just some Background for a nicer UI --&gt;
        &lt;Panel.Background&gt;
            ...
        &lt;/Panel.Background&gt;

        &lt;!-- This is our SnowFlakesControl. It can be used like any other Control. --&gt;
        &lt;controls:SnowflakesControl HorizontalAlignment="Stretch"
                                    VerticalAlignment="Stretch"
                                    IsRunning="{Binding IsGameRunning}"
                                    Snowflakes="{Binding Snowflakes}"
                                    Score="{Binding Score}" /&gt;

        &lt;!-- Add a Progressbar to indicate the left time  here --&gt;
        &lt;ProgressBar ... /&gt;

        &lt;!-- Add a Button the start command here --&gt;
        &lt;Button ... /&gt;
    &lt;/Panel&gt;
&lt;/UserControl&gt;</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
In <code>App.axaml</code> set <code>RequestedThemeVariant</code> to <code>"Dark"</code> for a better visibility of the white snowflakes.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>This is how the control will look like now (notice how the snowflakes disappear):</p>
</div>
<div class="paragraph">
<div class="title">First run output</div>
<p><span class="image"><img src="_docs/First_run.png" alt="First run"></span></p>
</div>
<div class="paragraph">
<p>Here is the final class <a href="Views/MainView.axaml">Views/MainView.axaml</a></p>
</div>
</div>
<div class="sect2">
<h3 id="_bonus_1_implement_a_custom_draw_operation">Bonus 1: Implement a custom draw operation</h3>
<div class="paragraph">
<p>If the provided render operations are not enough, you can create a new class which implements <code>ICustomDrawOperation</code>. In this sample we want to display the user score with a nice glow effect using the <a href="https://learn.microsoft.com/en-us/dotnet/api/skiasharp?view=skiasharp-2.88">[<code>SkiaSharp-API</code>]</a>.</p>
</div>
<div class="paragraph">
<p>The interface requires the following members to be implemented:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 6. <code>ICustomDrawOperation</code>-members</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Name</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Equals</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">bool</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">We can just return false here as we don&#8217;t want to reuse the class. Might be a good idea for a real-world App.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Dispose</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">void</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Use this in case you need to dispose or clean up anything.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">HitTest</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">bool</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Hit test the geometry in this node.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Render</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">void</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">This method will provide a <code>ImmediateDrawingContext</code> which gives you many low-level options to render.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Bounds</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Rect</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Gets the bounds of the visible content in the node in global coordinates.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>In addition to the interface members we will add one more property:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<caption class="title">Table 7. Properties of ScoreRenderer</caption>
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Name</th>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">Text</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">string</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Private readonly property which holds the text to render</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>Let&#8217;s see how the <code>Render</code>-method looks like:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public void Render(ImmediateDrawingContext context)
{
    // Try to get the skia-feature.
    var leaseFeature = context.TryGetFeature&lt;ISkiaSharpApiLeaseFeature&gt;();

    // In case we didn't find it, render the text with a fallback.
    if (leaseFeature == null)
    {
        var glyphs = Text.Select(c =&gt; Typeface.Default.GlyphTypeface.GetGlyph(c)).ToArray();

        var glyphRun = new GlyphRun(Typeface.Default.GlyphTypeface,
            20,
            Text.AsMemory(),
            glyphs,
            Bounds.TopRight - new Point(50, 50));

        context.DrawGlyphRun(Brushes.Goldenrod, glyphRun.TryCreateImmutableGlyphRunReference()!);
    }
    // Otherwise use SkiaSharp to render the text and apply some glow-effect.
    // Find the SkiaSharp-API here: https://learn.microsoft.com/en-us/dotnet/api/skiasharp?view=skiasharp-2.88
    else
    {
        using var lease = leaseFeature.Lease();
        var canvas = lease.SkCanvas;
        canvas.Save();

        using (var paint = new SKPaint())
        {
            paint.Shader = SKShader.CreateColor(SKColors.Goldenrod);
            paint.TextSize = 30;
            paint.TextAlign = SKTextAlign.Right;

            var origin = Bounds.TopRight.ToSKPoint();
            origin.Offset(-25, +50);

            paint.ImageFilter = SKImageFilter.CreateDropShadow(0, 0, 10, 10, SKColors.White);
            canvas.DrawText(Text, origin, paint);
        }
        canvas.Restore();
    }
}</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
SkiaSharp may be not always available. In this case it is highly recommended to provide a fallback.
</td>
</tr>
</table>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
You may see a warning about using an unstable API. This is okay to ignore, however you may need to adjust your code when you update to a newer Avalonia version.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>To consume the custom draw operation, add the following line into <code>SnowflakesControl.OnRender</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public override void Render(DrawingContext context)
{
    // [...]

    context.Custom(new ScoreRenderer(Bounds, $"Your score: {Score:N0}"));

    // [...]
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>If you now run the sample again, you can see the following outcome:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/Second_run.png" alt="Second run">
</div>
<div class="title">Figure 1. Second run output</div>
</div>
<div class="paragraph">
<p>Here is the final class <a href="Controls/ScoreRenderer.cs">Controls/ScoreRenderer.cs</a></p>
</div>
</div>
<div class="sect2">
<h3 id="_bonus_2_adding_some_beauty_to_the_game">Bonus 2: Adding some beauty to the game</h3>
<div class="paragraph">
<p>White snowflakes on a black screen may be boring for some users. In addition, the user can&#8217;t really see how much score each caught snowflake has gained. Let&#8217;s improve this.</p>
</div>
<div class="paragraph">
<p>In <code>MainView</code> we add a Background to the root <code>Panel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;Panel.Background&gt;
    &lt;LinearGradientBrush StartPoint="0,0" EndPoint="0%,100%"&gt;
        &lt;GradientStop Offset="0"   Color="DarkBlue" /&gt;
        &lt;GradientStop Offset=".85" Color="LightSkyBlue" /&gt;
        &lt;GradientStop Offset=".85" Color="LimeGreen" /&gt;
        &lt;GradientStop Offset="1"   Color="DarkGreen" /&gt;
    &lt;/LinearGradientBrush&gt;
&lt;/Panel.Background&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Moreover, we want to render some score hints for each hit snowflake. Similar to our <code>Snowflake</code>-class we will add the internal class <code>ScoreHint</code> which has the following properties:</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
This class can be internal as it is only meant to be consumed by the control itself.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>We won&#8217;t go into too much detail here as the concept is very close to the already discussed <code>Snowflake</code>-class. Only noticeable difference is that it has a reference to an <code>ICollection&lt;ScoreHint&gt;</code> in order to remove itself after the display time:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">internal void Update(double elapsedMilliseconds)
{
    // Increment total elapsed time
    _elapsedMillisecondsTotal += elapsedMilliseconds;

    // remove this item from the collection if it was there for 1 second (1000 ms)
    if (_elapsedMillisecondsTotal &gt;= 1000)
    {
        _scoreHintsCollection.Remove(this);
    }

    // after 500 ms we wipe out the opacity and move the text upwards
    if (_elapsedMillisecondsTotal &gt; 500)
    {
        var percentage = (_elapsedMillisecondsTotal - 500) / 500;
        Opacity = Math.Max(1.0 - percentage, 0); // Negative opacity doesn't work, so make sure it is always positive.
        Y -= (0.01 * percentage); // move slightly up.
    }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Here is the final class: <a href="Models/ScoreHint.cs">Models/ScoreHint.cs</a></p>
</div>
<div class="paragraph">
<p>We display the score hint in the <code>SnowflakeControl.OnRender</code>-method as follows:</p>
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<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public override void Render(DrawingContext context)
{
    foreach (var scoreHint in _scoreHintsCollection.ToArray())
    {
        // If the game is running, move each flake to it's updated position
        if (IsRunning)
        {
            scoreHint.Update(elapsedMilliseconds);
        }

        // Use a formatted text to render the score hint.
        var formattedText =
            new FormattedText(
                scoreHint.ToString(),
                CultureInfo.InvariantCulture,
                FlowDirection.LeftToRight,
                Typeface.Default,
                20,
                new SolidColorBrush(Colors.Yellow, scoreHint.Opacity));

        context.DrawText(formattedText, scoreHint.GetTopLeftForViewport(Bounds, new Size(formattedText.Width, formattedText.Height)));
    }
}</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_see_it_in_action">See it in action</h3>
<div class="paragraph">
<p>Hit [Run] in your IDE and enjoy your personal winter game.</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/final_run.png" alt="final run">
</div>
</div>
</div>
</div>
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<div class="sect1">
<h2 id="_related">Related</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="../../../../README.html#_drawing_samples">Samples.DrawingSamples</a></p>
</li>
<li>
<p><a href="https://github.com/AvaloniaUI/Avalonia/tree/master/samples/RenderDemo">Avalonia.RenderDemo</a></p>
</li>
</ul>
</div>
</div>
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Last updated 2025-11-07 18:43:23 UTC
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